Flavonoid accumulation and identification of flavonoid biosynthesis genes in Dimocarpus longan Lour. by transcriptome sequencing

Dimocarpus longan Lour. is widely cultivated and is very popular around the world. Its by-products such as roots and leaves have been used as traditional Chinese medicines due to their content of important secondary metabolites, especially flavonoids. However, the economic value and application of D. longan roots and leaves are limited because they accumulate relatively low levels of flavonoids. Therefore, it is important to find key genes that regulate the accumulation of the predominant flavonoid compounds in D. longan roots and leaves. Here, we have used RNA-sequencing to describe the transcriptome of D. longan. We obtained 75,229,529 raw reads and 15.04 GB of clean data, generating 56,055 unigenes (N50 = 1,583 nt, mean length = 829.61 nt). Next, we annotated these unigenes using the various available bioinformatics databases. By this approach, we identified 6,684 genes differentially expressed between root and leaf tissues, of which thirteen were identified as flavonoid biosynthesis genes. Of these, eight genes were much highly expressed in roots (DlC4H, DlHCT, DlDFR, DlANS, DlANR, DlCHS, DlF3'H, and DlF3H), and two were much highly expressed in leaves (DlLAR and DlFLS). The contents of thirteen flavonoids in D. longan roots and leaves were measured by LC-MS, and epicatechin was found to be the predominant flavonoid in both tissues, which was significantly higher than the other flavonoids measured in the study. Its contents were 213,773.65 ng/g in roots and 22,388.71 ng/g in leaves. Our findings will facilitate efforts to increase the economic value and expand the applications of D. longan roots and leaves by means of genetic engineering.